I'd like to offer a different experience regarding driving in Italy. We spent a wonderful 10 day vacation in Tuscany, renting a car at Rome FCO and returning it there.
First of all, I agree with Roberto. You need to do some more research. You don't have much time in the region and you need to get a clear idea of what you want to see. If you plan to spend almost all of your time in Siena, San Gimignano and Montalcino, renting a car doesn't make a lot of sense because you'll be parking it outside the towns and walking in the ZTL areas Ken mentioned. Also, if you want to include a visit to a winery or two in the Tuscan countryside, unless you have a designated driver, taking a small tour or hiring a car and driver may make more sense.
OTOH, we loved the freedom afforded by having our own car. This allowed us to keep our schedule fluid and explore back roads and small towns we would not have seen otherwise. There's a big difference between seeing something interesting from a bus as you whiz by and seeing something interesting (Etruscan ruins, for example) and being able to stop and explore for as long as you like.
Finally, I'd like to address driving in Italy for you and any others who may read this thread looking for information. What kind of driver are you? If you are an assured defensive driver who doesn't rack up speeding tickets and don't have frequent accidents, you should have no problems driving in Italy if you do a little homework before you leave.
Yes, many cities and towns have ZTL's. They are very well marked and anyone who does some homework on signage and studies maps should have no problems avoiding them.
Yes, the Autostrada has the Tutor system (this should not be a factor for you). The Autostrade is a toll road and it also has cameras placed at intervals to calculate average time. Observe the speed limit and you won't have any problems. We also took frequent stops at the wonderful Autogrills where we enjoyed a real cup of coffee or an excellent Panini. This helps with the average speed.
There are speed cameras in the countryside and in towns. Again, just observe the speed limits and you'll be fine. The intent is not to rush from one place to another, but to enjoy the scenery. Every once in a while there would be a local driver who wanted to go faster than we did. I'd just find the closest place turn off and let them pass. Most of the time I'd get a wave of appreciation.